Many adhesives used to bond or laminate plastic film, such as polyolefin film, to metals are not water-based. These adhesives frequently use solvents other than water, e.g. organic solvents, and the like, and they are often flammable and not environmentally friendly. In these adhesives, the solvent serves as the carrier for the active adhesive components which are dissolved or dispersed in it and this solvent must be driven away during the application and lamination process. Removal of these solvents poses concerns to the environment and requires handling precautions stemming from the flammability of the solvents. In addition, cleaning adhesives that are not water-based from equipment can be difficult and time consuming. Many processes for bonding plastic films to metal surfaces involve other steps that need to be performed prior to the lamination step. Such steps may include applications of inorganic conversion coatings such as phosphate pretreatments; application and drying of primer layers such as solvent-based primers, and application and drying of the adhesives. Following these pretreatment steps, the plastic film is then laminated to the layered metal surface either using a preformed sheet/film of plastic or the plastic is extruded onto the adhesive coated metal substrate. In such cases, the total thickness of the preparation layers between the metal and the plastic film may range from 8 to 40 microns or more. For many applications using these prior art adhesives these multiple coating layers are required to obtain a proper balance between adhesion of the film to the metal surface and corrosion resistance of the laminate.
In the art of coiled metal rolls, it is very desirable to have an adhesive that can be applied to the metal prior to coiling it wherein the adhesive coated coiled metal can later be unrolled and used to laminate a plastic film to the metal. One hindrance has been that many current adhesives exhibit a phenomena know in the art as blocking. In the context of coiled metal, the term blocking refers to the effect many adhesives have of bonding the metal to itself such that the coil can not be uncoiled. This effect occurs when the still warm metal, having an adhesive coating, is rolled into a coil. The warm metal causes sufficient activation of the adhesive such that the coil can not be subsequently unrolled because it is stuck onto itself by the adhesive. This is known in the art of coiled metal as blocking. One current way to prevent blocking is to include additional extreme cooling steps after application of the adhesive to prevent its activation. Blocking is more severe with adhesive polymers having low softening temperatures because the metal temperature during the rolling step is often near to the polymer softening temperature. Thus, many desirable adhesive polymers can not be used on metal that will be coiled unless one includes additional quick cooling steps. Blocking is also a problem in the industry because often it is desirable to take the metal substrate, coat it with an adhesive layer and then roll into a coil for transport to a different location where it may under go further treatment followed by lamination of a plastic film onto the substrate. In such a process, if the adhesive causes blocking then the process can not be followed, without extra cooling steps, because the coil once formed can not be unrolled.
It is desirable to create an adhesive composition that does not rely on non-water solvents such as those currently in use because of the environmental, clean up, and flammability issues of these solvents. To that end it would be desirable to develop a water-based adhesive composition that would have minimal impact on the environment, be easy to clean up and be non-flammable. In addition, it is desirable to create an adhesive composition that can be used in thinner application layers and preferably still provide both adhesion and corrosion resistance. In addition, it is desirable to create an adhesive composition that can be applied to a warm metal substrate, dried in place and then the substrate recoiled while the metal is still warm with reduced or no blocking effect. It is also desirable to provide an adhesive composition that can accommodate addition of adhesive polymers that have low softening temperatures without leading to blocking in coiled metal rolls that are recoiled while warm. Finally, it is desirable to reduce the cost of adhering plastic films to metals and to reduce processing time.